The present invention relates to a fuel cell and, more particularly, to a system and method for compensating a relative humidity signal output by a relative humidity sensor of a fuel cell for variations in pressure, temperature and gas composition.
Fuel cells are increasingly being used as a power source in a wide variety of different applications. Fuel cells have also been proposed for use in electrical vehicular power plants to replace internal combustion engines. A solid-polymer-electrolyte fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H2) is supplied to the anode and air or oxygen (O2) is supplied to the cathode.
In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (exe2x88x92). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane while the electrons flow through an electrical load that is connected across the electrodes. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+), and electrons (exe2x88x92) are taken up to form water (H2O).
To operate efficiently and to produce a maximum amount of electricity, the fuel cell must be properly humidified. To achieve the proper humidity range, the hydrogen stream and/or the air stream are typically humidified by one of several methods known in the art. Conventional humidity control methods generally fail to sufficiently control the humidity of the hydrogen and air streams to the fuel cell. Providing too much humidity to the fuel cell blocks the reacting gases from accessing the catalyst thereby impeding the electrochemical reaction between the hydrogen and air and reducing the production of electricity. Providing too little humidity to the fuel cell restricts or limits the proton transportation required for reaction within the fuel cell.
In U.S. patent applications Ser. No. 09/491,308 now 6,376,111, entitled xe2x80x9cSystem and Method for Controlling the Humidity Level of a Fuel Cellxe2x80x9d, which is hereby incorporated by reference, a controller 44 utilizes feedback to control the humidity of the fuel cell assembly. The resistance of the fuel cell assembly (as measured across the DC/DC converter) is used to control the humidity of the fuel cell assembly.
Relative humidity sensor readings are needed at the inlet of the anode and the cathode so that the humidity level can be accurately determined and controlled. Commercially available relative humidity sensors are typically calibrated for operation in a specific medium such as air, for a particular temperature range, and for a particular pressure range such as atmospheric pressure. When these relative humidity sensors are placed in pressurized air, reformate and/or hydrogen streams of a fuel cell, the resulting measurement of the relative humidity is no longer accurate. Failure to properly control the relative humidity of the fuel cell leads to the problems described above. The commercially available relative humidity sensors have not been calibrated to handle variations in temperature, pressure and gas composition that are likely to be encountered in fuel cells. Development of sensors that are accurate despite the variations in operating conditions has been difficult and expensive. The cost of the humidity sensor becomes more important when the fuel cells are used to power a motor vehicle. One major obstacle limiting the commercial feasibility of fuel cells in motor vehicles is cost.
A system for sensing the humidity level of a fuel cell according to the invention includes a humidity sensor that senses the relative humidity of a gas stream and generates a relative humidity signal. A first sensor senses pressure or temperature of the gas stream and generates a temperature signal or a pressure signal. A compensator is connected to the humidity sensor and the first sensor and generates a compensated relative humidity signal based on the relative humidity signal and the temperature signal or the pressure signal.
According to other features of the invention, the system further includes a humidifier. A controller is connected to the compensator and the humidifier. The controller increases the relative humidity of the gas stream based on the compensated relative humidity signal.
According to still other features of the invention, the gas stream is provided by a reformate source or a hydrogen source to an anode of the fuel cell. Alternately, the gas stream is provided by an air source or an oxygen source to a cathode of the fuel cell.
In other features of the invention, the compensator includes memory containing look-up tables and/or mathematical formulas that are used to determine the compensated relative humidity signal.
In still other features of the invention, a gas composition sensor senses the concentration of a first gas in the gas stream and generates a first gas concentration signal. The compensator is connected to the gas composition sensor. The compensated relative humidity signal is based on the relative humidity signal, the temperature signal or the pressure signal, and the first gas composition signal.
Still other objects, features and advantages will be readily apparent to skilled artisans from the specification, the claims and the drawings.